Fig. 1.

PolyP granule formation in Pseudomonas aeruginosa under nitrogen starvation. Each panel is a 20-nm-thick tomographic slice through a 3D reconstruction of an intact cell. (Scale bar, 200 nm.) (A) Cryotomogram of a WT cell 1 h after inducing starvation. The cell shows many microgranules of polyP (black arrowheads). (B) WT cell 3 h after inducing starvation. Microgranules consolidate within the nucleoid region into regularly spaced mature granules. (C) WT cell 24 h after inducing starvation. PolyP granules lose tight spatial organization, and they are of heterogeneous size and number. PHA granules (white arrowheads) are formed late in starvation. (D) ∆polyP cell 1 h after inducing starvation. Quadruple knockout of polyphosphate kinases ∆ppk1∆ppk2a∆ppk2b∆ppk2c. (E) ∆polyP cell 3 h after inducing starvation do not form polyP granules but form PHA granules earlier than WT. (F) ∆polyP cell 24 h after inducing starvation contain PHA granules and are elongated, with some showing evidence of initiation but not completion of septation. (G) (Top) Model of steps in cell cycle exit. Reinitiation of DNA replication is inhibited. Replication elongation rates are modulated for efficiency to prevent stalled forks and fork breakage. Daughter chromosomes are segregated and compacted. Septation proceeds in the absence of stalled forks and once daughter chromosomes are segregated. (Bottom) Model for steps in polyP granule genesis. PolyP granules initiate throughout the nucleoid region. Granule number decreases, whereas size increases, by some combination of dilution of granules due to cell division and/or granule consolidation. Mature granules become transiently spatially organized. Granule number, size distribution, and positioning relax deeper into starvation.